電腦攻擊與防禦 The Attack and Defense of Computers Dr. 許 富 皓

1. 電腦攻擊與防禦 The Attack and Defense of Computers Dr.許 富 皓

2. Attacking Program Bugs

3. Attack Types • Buffer Overflow Attacks: • Stack Smashing attacks • Return-into-libc attacks • Heap overflow attacks • Function pointer attacks • .dtors overflow attacks. • setjump/longjump buffer overflow attacks. • Format string attacks: • Integer overflow and integer sign attacks

4. Why Buffer Overflow Attacks Are So Dangerous? • Easy to launch: • Attackers can launch a buffer overflow attack by just sending a craft string to their targets to complete such kind of attacks. • Plenty of targets: • Plenty of programs have this kind of vulnerabilities. • According to CERT, more than 50% of today’s Internet incidents are launched through buffer overflow attacks. • Cause great damage: • Usually the end result of a buffer overflow attack is the attacker’s gaining the root privilege of the attacked host. • Internet worms proliferate through buffer overflow attacks.

5. Stack Smashing Attacks

6. Principle of Stack Smashing Attacks • Overwritten control transfer structures, such as return addresses or function pointers, to redirect program execution flow to desired code. • Attack strings carry both code and address(es) of the code entry point.

7. Explanation of BOAs (1) G(int a) { H(3); add_g: } H( int b) { char c[100]; int i; while((c[i++]=getch())!=EOF) { } } G’s stack frame b return address add_g H’s stack frame address of G’s frame point C[99] 0xabc Z Y X 0xabb Input String: xyz C[0] 0xaba

8. Explanation of BOAs (2) Length=108 bytes G(int a) { H(3); add_g: } H( int b) { char c[100]; int i; while((c[i++]=getch())!=EOF) { } } Attack String: xxInjected Codexy0xabc b return address add_g addrress oxabc H’s stack frame address of G’s frame point y C[99] x Injected Code 0xabc 0xabb x x C[0] 0xaba

9. Injected Code: • The attacked programs usually have root privilege; therefore, the injected code is executed with root privilege. • The injected code is already in machine instruction form; therefore, a CPU can directly execute it. • However the above fact also means that the injected code must match the CPU type of the attacked host. • Usually the injected code will fork a shell; hence, after an attack, an attacker could have a root shell.

10. Injected Code of Remote BOAs • In order to be able to interact with the newly forked root shell, the injected code usually need to execute the following two steps: • Open a socket. • Redirect standard input and output of the newly forked root shell to the socket.

11. Example of Injected Code for X86 Architecture : Shell Code • char shellcode[] ="\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd\x80\xe8\xdc\xff\xff\xff/bin/sh";

12. Two Factors for A Successful Buffer Overflow-style Attack(1) • A successful buffer overflow-style attack should be able to overflow the right place (e.g. the place to hold a return address with the correct value (e.g. the address of injected code entry point)).

13. Two Factors for A Successful Buffer Overflow-style Attack(2) return address buffer where the overflow start injected code address of injected code entry point. offset between the beginning of the overflowed buffer and the overflow target. The offset and the entry point address are non-predicable. They can not decided by just looking the source code or local binary code.

14. Non-predicable Offset • For performance concerns, most compilers don’t allocate memory for local variables in the order they appear in the source code, sometimes some space may be inserted between them. (Source Code doesn’t help) • Different compiler/OS uses different allocation strategy. (Local binaries don’t help) • Address obfuscation insert random number of space between local variables and return address. (Super good luck may help)

15. Function main()’s stack frame Non-predicable Entry Point Address webserver –a –b security [fhsu@ecsl]# system data 0xbfffffff environment variables argument strings command line arguments and environment variables env pointers argv pointers argc

16. Strategies Used by Attackers to Increase Their Success Chance • Repeat address patterns. • Insert NOP (0x90) operations before the entry point of injected code.

17. Exploit Code Web Sites • Exploit World • The Metasploit Project

18. An Exploit Code Generation Program • This program uses the following three loop to generate the attack string which contains the shell code. for(i=0;i<sizeof(buff);i+=4) *(ptr++)=jump; for(i=0;i<sizeof(buff)-200-strlen(evil);i++) buff[i]=0x90; for(j=0;j<strlen(evil);j++) buff[i++]=evil[j];

19. Return-into-libc Attacks

20. Return-into-libc • A mutation of buffer overflow attacks. • Utilize code already resided in the attacked programs’ address space, such as libc functions. • Attack strings carry entry point address(es) of a desired libc function, new frame point address and parameters to the function.

21. How Parameters and Local Variables Are Represented in An Object file? abc(int aa) { int bb; bb==aa; : : } abc: function prologue *(%ebp-4)=*(%ebp+8) function epilogue aa return address previous frame point ebp bb

22. A Way to Change The Parameters and Local Variables of A Function. • A parameter or a local variable in an object file is represented through its offset between the position pointed by %ebp and its own position. • Therefore, the value of the %ebp register decides where a function to get its parameters and local variables. • In other words, if an attacker can change the %ebp of a function, then she/he can also change the function’s parameters and local variables.

23. Function Prologue and Epilogue #include <stdio.h> int add_three_items(int a, int b, int c) { int d; d=a+b+c; return d; } add_three_items: pushl %ebp movl %esp, %ebp subl $4, %esp movl 12(%ebp), %eax addl 8(%ebp), %eax addl 16(%ebp), %eax movl %eax, -4(%ebp) movl -4(%ebp), %eax leave ret 3 function prologue 4 function epilogue leave=movl %ebp,%esp popl %ebp

24. Function Calls main() { int a, b,c,f; extern int add_three_items(); a=1; b=2; c=3; f=add_three_items(a,b,c); } main: pushl %ebp movl %esp, %ebp subl $24, %esp andl $-16, %esp movl $0, %eax subl %eax, %esp movl $1, -4(%ebp) movl $2, -8(%ebp) movl $3, -12(%ebp) subl $4, %esp pushl -12(%ebp) pushl -8(%ebp) pushl -4(%ebp) call add_three_items addl $16, %esp movl %eax, -16(%ebp) leave ret 1 2 5 leave=movl %ebp,%esp popl %ebp

25. function: pushl %ebp movl %esp, %ebp subl $40, %esp leave ret main: pushl %ebp movl %esp, %ebp subl $8, %esp andl $-16, %esp movl $0, %eax addl $15, %eax addl $15, %eax shrl $4, %eax sall $4, %eax subl %eax, %esp pushl $3 pushl $2 pushl $1 call function addl $12, %esp leave ret • Example code void function(int a, int b, int c) { char buffer1[5]; char buffer2[10]; } main(int argc, char *argv[]) { function(1,2,3); } gcc -S test.c;

26. bp sp high ret addr (EIP) function: pushl %ebp movl %esp, %ebp subl $40, %esp leave ret main: pushl %ebp movl %esp, %ebp subl $8, %esp andl $-16, %esp movl $0, %eax addl $15, %eax addl $15, %eax shrl $4, %eax sall $4, %eax subl %eax, %esp pushl $3 pushl $2 pushl $1 call function addl $12, %esp leave ret %ebp … $3 $2 $1 ret addr (EIP) %ebp … leave = mov %ebp, %esp pop %ebp heap bss low

27. Explanation of Return-into-libc G(int a) { H(3); add_g: } H( int b) { char c[10]; overflow occurs here } parameter 1, e.g. pointer to /bin/sh b any value return address add_g abc(), e.g. system() H’s stack frame address of G’s frame point any value esp ebp C[9] C[0] abc: pushl %ebp movl %esp,%ebp

28. Explanation of Return-into-libc G(int a) { H(3); add_g: } H( int b) { char c[10]; overflow occurs here } parameter 1, e.g. pointer to /bin/sh b any value return address add_g abc(), e.g. system() H’s stack frame address of G’s frame point any value esp ebp C[9] movl %ebp,%esp (an instruction in function epilogue) C[0] abc: pushl %ebp movl %esp,%ebp

29. Explanation of Return-into-libc G(int a) { H(3); add_g: } H( int b) { char c[10]; overflow occurs here } parameter 1, e.g. pointer to /bin/sh b any value return address add_g abc(), e.g. system() esp H’s stack frame address of G’s frame point (popl %ebp) any value ebp any value C[9] C[0] abc: pushl %ebp movl %esp,%ebp

30. Explanation of Return-into-libc G(int a) { H(3); add_g: } H( int b) { char c[10]; overflow occurs here } parameter 1, e.g. pointer to /bin/sh b esp any value return address add_g (ret) abc(), e.g. system() H’s stack frame address of G’s frame point any value ebp any value C[9] C[0] abc: pushl %ebp movl %esp,%ebp

31. Explanation of Return-into-libc After the following two instruction in function system()’s function prologue is executed pushl %ebp movl %esp, %ebp, the position of %esp and %ebp is shown in the figure. G(int a) { H(3); add_g: } H( int b) { char c[10]; overflow occurs here } parameter 1, e.g. pointer to /bin/sh b any value return address add_g ebp any value esp H’s stack frame address of G’s frame point any value C[9] C[0] abc: pushl %ebp movl %esp,%ebp

32. Properties of Return-into-libc Attacks • The exploit strings don’t need to contain executable code.

33. Heap/Data/BSS Overflow Attacks

34. Principle of Heap/Data/BSS Overflow Attacks • Similarly to stack smashing attacks, attackers overflow a sensitive data structure by providing a buffer which is adjacent to the sensitive data structure more data than the buffer can store; hence, to overflow the sensitive data structure. • The sensitive data structure may contain: • A function pointer • A pointer to a string • … and so on. • Both the buffer and the sensitive data structure may locate at the heap, or data, or bss section.

35. Heap and Data/BSS Sections • The heap is an area in memory that is dynamically allocated by the application by using a system call, such as malloc() . • On most systems, the heap grows up (towards higher addresses). • The data section initialized at compile-time. • The bsssection contains uninitialized data, and is allocated atrun-time. • Until it is written to, it remains zeroed (or at least from the application's point-of-view).

36. Heap Overflow Example #define BUFSIZE 16 int main() { int i=0; char *buf1 = (char *)malloc(BUFSIZE); char *buf2 = (char *)malloc(BUFSIZE); : while((*(buf1+i)=getchar())!=EOF) i++; : }

37. BSS Overflow Example #define BUFSIZE 16 int main(int argc, char **argv) { FILE *tmpfd; static char buf[BUFSIZE], *tmpfile; : tmpfile = "/tmp/vulprog.tmp"; gets(buf); tmpfd = fopen(tmpfile, "w"); : }

38. BSS and Function Pointer Overflow Example int goodfunc(const char *str); int main(int argc, char **argv) { int i=0; static char buf[BUFSIZE]; static int (*funcptr)(const char *str); : while((*(buf+i)=getchar())!=EOF) i++; : }

39. Function Pointer Attacks

40. Principle of Function Pointer Attacks • Utilizing a function pointer variable’s adjacent buffer to overwrite the content of the function pointer variable so that it will point to the code chosen by attackers. • A function pointer variable may locate at the stack section, the data section, or at the bss section.

41. Countermeasures of Buffer Overflow Attacks

42. Countermeasures of Buffer Overflow Attacks (1) • Array bounds checking. • Non-executable stack/heap. • Safe C library. • Compiler solutions, e.g., • StackGuard • RAD • Type safe language, e.g. Java. • Static source code analysis.

43. Countermeasures of Buffer Overflow Attacks (2) • Anomaly Detection, e.g. through system calls. • Dynamic allocation of memory for data that will overwrite adjacent memory area. • Memory Address Obfuscation • Randomization of executable Code. • Network-based buffer overflow detection

44. Array Bounds Checking • Fundamental solution for all kinds of buffer overflow attacks. • High run-time overhead (33 times in some situations)

45. Non-executable Stack/Heap • The majority of buffer overflow attacks are stack smashing attacks; therefore, a non-executable stack could block the majority of buffer overflow attacks. • Disable some original system functions, e.g. signal call handling.

46. Safe C Library • Some string-related C library functions, such as strcpy and strcat don’t check the buffer boundaries of destination buffers, hence, modifying these kinds of unsafe library functions could secure programs that use these function. • Replace strcpy with strncpy, or replace strcat with strncat, … and so on. • Plenty of other C statements could still results in buffer overflow vulnerabilities. • E.g. while ((*(ptr+i)=getchar())!=EOF) i++;

47. Compiler Solutions: StackGuard • Put a canary word before each return address in each stack frame. Usually, when a buffer overflow attack is launched, not only the return address but also the canary word will be overwritten; thus, by checking the integrity of the canary word, this mechanism can defend against stack smashing attacks. • Low performance overhead. • Change the layout of the stack frame of a function; hence, this mechanism is not compatible with some programs, e.g. debugger. • Only protect return addresses.

48. Compiler Solutions: RAD • Store another copies of return addresses in a well-protected area, RAR. • When a function is call, instead of saving its return address in its corresponding stack frame, another copy of its return address is saved in RAR. When the function finishes, before returning to its caller, the callee checks the return address in its stack frame to see whether the RAR has a copy of that address. If there is no such address in the RAR, then a buffer overflow attack is alarmed. • Low performance overhead. • Only protect return addresses.

49. Type Safe Language, e.g. Java • These kinds of languages will automatically perform array bound checking. • The majority of programs are not written in these kinds of languages; rewriting all programs with these kinds of languages becomes an impossible mission.

50. Static Source Code Analysis. • Analyze source code to find potential program statements that could result in buffer overflow vulnerabilities. E.g. program statements like while((*(buf+i)=getchar())!=EOF) i++; are not safe. • False positive and false negative. • Difficulty to obtain the source code.